Thin film deposition is essential to the manufacture of solid state electronic devices. By layering various materials (i.e., films) on a wafer in a prescribed pattern, a solid state electronic device is formed.
Within the semiconductor device industry there is an ever present trend for more complex multi-layer structures and smaller device dimensions. In order to reduce the lateral device area of storage capacitors, for example, high aspect ratio (i.e., high depth to width ratio) features (e.g., steps, trenches and vias) have become prevalent. Such features possess large side wall surface areas which allow lateral device dimensions to shrink while maintaining constant capacitor area (and thus a constant capacitance).
When depositing a film over a high aspect ratio feature, material tends to deposit near the top surface (i.e. the surface nearest the deposition material source or target) of the feature and to prevent subsequently deposited material from reaching the feature's lower surfaces, causing variations in deposition layer thickness including voids (areas containing no deposition material). Accordingly, much attention has been directed to formation of continuous conformal films within high aspect ratio features, and/or to continuous filling of high aspect ratio features (e.g., aluminum planarization).
As described in "Aluminum Planarization for Advanced Via Applications," European Semiconductor, February 1996, a preferred technique for achieving aluminum planarization is known as the high temperature flow process (i.e., reflow). Aluminum reflow typically begins with deposition of a thick film of aluminum that is deposited on a wafer at relatively low wafer temperatures (e.g., less than 150.degree. C.). The wafer is then transferred to a reflow chamber and heated to a temperature at which the aluminum film flows (i.e., a reflow temperature). A temperature of 570.degree. C. is conventionally required to supply aluminum with sufficient energy to cause the aluminum to flow. At this temperature solid phase diffusion and surface tension transport aluminum across the wafer's surface, and transport aluminum from the top region of surface features to the bottom of surface features (i.e., top down filling).
Although conventional reflow methods can produce adequate filling of many types of surface features, reflow is unfeasible for small feature applications. Thus, for small feature applications a more costly and time consuming process known as cold/hot sequential deposition (having wafer temperatures in the range of 430-475.degree. C.) must be employed.
Moreover, both conventional reflow and cold/hot sequential deposition processes experience degraded film properties caused by contaminants (water vapor, O.sub.2, etc.) which desorb from chamber surfaces, from process kit parts, or from the wafer itself during the planarization process. These contaminants enter the chamber atmosphere and are pumped therefrom. Over time fewer contaminants desorb, however, water is often the last contaminant to desorb, and can break into hydrogen and oxygen. Free oxygen is highly reactive and tends to bond with the film (e.g., aluminum) causing the film to surface passivate (e.g., tying up dangling bonds and reducing the surface energy of the film). Surface passivation degrades the film's ability to flow (i.e., degrades surface mobility), thus encouraging void formation. In addition to degrading surface mobility of the film, oxygen and hydrogen, because they have very low atomic weights, are extremely difficult to pump from the chamber. These contaminants remain in the chamber, producing an increase in chamber base pressure which causes the film to have poor crystal orientation and film purity. The poor crystal orientation and film purity result in resistivity and reflectivity deterioration. Thus contaminants which desorb from processed wafers can increase chamber base pressure, and consequently, cause incomplete feature filling due to decreased surface mobility.
Accordingly, a need exists for an expeditious and cost effective method and apparatus for planarizing films at relatively low temperatures, and for a method which resists the adverse affects of desorbed contaminants.